This invention relates to coating compositions formulated in part from certain polyols modified by reaction with a glycidyl ether. In particular, it pertains to compositions so formulated which have a relatively high solids content, which can be thinnable or reducible with water and can be completely free of water immiscible organic solvents and which are especially useful for the top coating of surfaces or rigid wood or wood-like products, such as paneling or the like, to enhance the durability and appearance thereof.
Clear topcoats for the finishing of wood and flat woodstock products used in paneling, furniture, etc. have conveniently been formulated from alkyd resins dissolved or dispersed in a non-aqueous liquid vehicle which is or contains an organic hydrocarbon solvent. In the flat woodstock industry such products are normally produced by the pressing together of cellulosic particles and chips to form a composite structure which is then treated with an appropriate sealer and printed with a suitable ink to provide a wood-like grain. A clear topcoat is then applied to the article to render its surface more attractive and to make it durable and resistant to abrasion, scratching, the action of water and solvent-based cleaners etc. The conventional organic-solvent-based alkyd resin topcoats have provided these properties and have adhered well to the wood or woodstock articles, as well as, any printing inks present therein.
However, adverse environmental effects of organic solvents, particularly hydrocarbon solvents, including air and water pollution effects, photochemical degradation effects and even adverse health effects attributed to certain hydrocarbon solvents, as well as high cost of solvents, have led the coatings industry to seek to develop aqueous coating compositions free of hydrocarbon solvents and which possess all the desirable attributes of conventional hydrocarbon solvent-based coatings.
The development of clear water-based topcoats for finishing of wood, and especially woodstock products, which are as suitable for the purpose as alkyd resin systems, has been particularly difficult for a number of reasons. The coating composition as supplied to users should be stable on storage in summer and winter without crystallization of material from the aqueous medium even in cold climate when stored outside, or increasing in viscosity. It should be of relatively high solids content but be reducible or thinnable with water to provide as thin a coating as desired. It should be capable of application to wood or woodstock paneling without gellation for many hours or phasing out from the aqueous solvent or other difficulties during conventional coating, and of being subjected after application to a rapid, relatively low temperature bake to remove the volatile components and effect a cure to the desired finished coating. The coating applied should not be blemished or marred when the paneling is stacked on itself after being coated and it should adhere to the substrate (and any printing inks present to provide wood-like grain), so tenaciously as not to be removable under any use conditions, as well as having the requisite durability and resistance.
Prior to this invention, there has been developed a specific type of water-based composition for clear topcoating of wood and woodstock products which possesses some of the desired attributes. This composition is formulated using active film-forming ingredients trimethylol ethane and a water soluble or dispersible methylated melamine-formaldehyde condensate. The trimethylol ethane functions uniquely in the composition, since other simple triols such as trimethylol propane, glycerol etc. are unsuitable. However, even this specific type of composition has been found lacking for the desired purpose. When the composition is stored at lower temperatures, undesirable crystallization sometimes occurs and then coatings applied therefrom to woodstock paneling do not possess sufficient adhesion to the substrate to pass an adhesive "tape test" to be hereinafter described.
It is also known from U.S. Pat. Nos. 3,959,201 and 3,994,581 that high solids, aqueous-solvent-thinnable coating compositions useful for a variety of purposes, but not especially designed for top coating of wood and wood-like surfaces, can be formulated in aqueous media by combining various polyols (i.e., organic materials containing a plurality of alcoholic hydroxyl groups), including trimethylol ethane, with aminoplast resins.